The present invention concerns a process for the production of aminoplastic resins. Its object is more particularly a process for the production of aminoplastic resins that enables chipboard to be produced that gives off little formaldehyde and that has improved mechanical properties.
The chipboard is usually produced by hot pressing a mass of wood chips or other cellulosic matter mixed with binders, in particular solutions of urea-formaldehyde or melamine-urea-formaldehyde resins. The pressing temperatures are generally about 150.degree. to 220.degree. C. so as to obtain good adhesion of the chips in an acceptable production time, using high pressure up to 70 bars.
The main drawback of aminoplastic resins is that they give off large amounts of free formaldehyde. Attempts have been made to reduce the free formaldhyde content by developing various production processes. Unfortunately, when particularly low formaldehyde contents are aimed at, experience shows that this objective is accompanied by a deterioration in the mechanical properties of the finished boards. It has also been proposed that the presence of formaldehyde could be avoided by using formaldehyde-free resins, in particular resins based on isocyanate solutions. Unfortunately the problems associated with formaldehyde are replaced by those associated with isocyanates, whose action on man is more harmful and more durable than that of formaldehyde since the presence of free isocyanate groups has been demonstrated in the boards many years after they were produced. Moreover the use of such resins in molding presses poses serious de-moulding problems due to the affinity of isocyanates for metallic components. It has also been proposed that processes should be used comprising spraying the two binders separately onto the chips: such processes are awkward due to the low proportion of isocyanate that may economically be used.
The need is therefore felt for aminoplastic resins to be developed having low formaldehyde contents and whose use produces finished boards emitting little formaldehyde, and possessing good mechanical characteristics.